The present invention relates generally to communication systems and, more particularly, to wireless communication systems adapted to use Orthogonal Frequency Division Multiplexing (OFDM) modulation techniques. As used herein, the abbreviations set forth below shall have the meanings adjacent thereto:
3G LTE Third Generation Long Term Evolution
CPE Common Phase Error
ICI Inter Carrier Interference
IF Intermediate Frequency
FDM Frequency Division Multiplexing
FFT Fast Fourier Transform
LTE Long-Term Evolution
LO Local Oscillator
MCM Multi-Carrier Modulation
OFDM Orthogonal Frequency Division Multiplexing
PNL Phase Noise Level
WLAN Wireless Local Area Network
Multi-Carrier Modulation (MCM) is the principle of transmitting data by (i) dividing the stream into several parallel bit streams, each of which has a much lower bit rate, and (ii) using these substreams to modulate several carriers. The first systems using MCM were military HF radio links in the late 1950s and early 1960s. Orthogonal Frequency Division Multiplexing (OFDM), is a special form of MCM with densely spaced sub-carriers and overlapping spectra. OFDM abandoned the use of steep bandpass filters that completely separated the spectrum of individual sub-carriers, as was common practice in older Frequency Division Multiple Access (FDMA) systems. Rather, OFDM time-domain waveforms are chosen such that mutual orthogonality is ensured even though sub-carrier spectra may overlap. OFDM relies on the insight in communications provided by Shannon, in particular, in his “geometric” theory, considering waveforms to be a point in a Euclidean space, allowing definitions of orthogonality. OFDM multiplexes signals by dividing the available bandwidth into a series of frequencies known as sub-carriers. In OFDM, the data is transmitted on a large number of orthogonal sub-carriers, where the frequency separation between the sub-carriers equals the reciprocal of the duration of an OFDM symbol. OFDM achieves high performance with reasonable complexity at the receiver.
Evolving mobile cellular standards such as WiMAX, WLAN, and 3G Long Term Evolution (3G LTE) will likely require modulation techniques such as OFDM in order to deliver higher data rates. An advantage of OFDM is its inherent robustness to time-dispersive channels, as the duration of an OFDM symbol can be made large even when the supported data rate is high.
In order for OFDM to work properly, however, it is important that the different sub-carriers that are orthogonal when transmitted also are orthogonal at the receiver. There are a number of reasons why the sub-carriers will no longer be perfectly orthogonal at the receiver. When the sub-carriers are no longer orthogonal at the receiver, information transmitted on the sub-carrier to some extent interferes with the adjacent sub-carriers. This interference is commonly referred to inter-carrier interference (ICI).
ICI may occur because the channel is varying due to the movement of the transmitter, receiver, or both. ICI may also occur due to imperfections at both the transmitter and the receiver. One such imperfection is frequency error based on an offset between the transmitter and the receiver. Frequency error can often be made sufficiently small if the frequency can be estimated for a sufficient period of time. Another imperfection is phase noise. As opposed to a frequency error, which can be considered static, the phase noise changes from one OFDM symbol to the next. In fact, for OFDM systems the degradation due to phase noise is sometimes addressed by dividing it into two parts, one that is caused by the low-frequency part, and one which is caused by the high frequency part. The border between the low frequency and high frequency equals half the sub-carrier spacing.
Heterodyne receivers include a mixer in the receiver chain to convert the incoming high frequency RF signal to an intermediate frequency (IF), where much improved selectivity is possible. A local oscillator (LO) is required to implement this design. The difference between the LO frequency and input signal frequency produces the desired IF. By making the IF a fixed frequency and tuning the LO to select a given channel, the IF selection filter and additional amplification can be carefully optimized for good selectivity with small size and low cost. A special case of the heterodyne receiver is the zero IF receiver, also known as the homodyne receiver, where the LO coincides with the incoming carrier frequency giving an IF of zero Hz. This is also called a direct conversion receiver.
Because the signal in a zero IF design is mixed down to about zero Hz, quadrature versions of the signal must be generated in order to allow the detector to differentiate between in-band components above the LO frequency and those below the LO frequency.
In a direct conversion receiver, there are few image problems, and the IF selection filter becomes a pair of low pass filters at baseband. This allows the filter to have even greater selectivity with better gain and phase response. Direct conversion receivers are often used in conjunction with a digital signal processor to implement the channel filtering and data detection. In this case, the two baseband analog outputs from the mixers are digitized using A/D converters. The advantage of using DSP to implement channel filtering is that near perfect gain and phase response can be realized with very high order (highly selective) filters with variable passband. This in turn means that the channel spacing can be changed under software control.
There are several disadvantages with direct conversion receivers. The first is the problem of local oscillator re-radiation from the antenna, as the LO, which is at the frequency of the incoming signal, may leak back through the front end mixer/amplifier/filter chain. A second disadvantage is DC-offset within the two baseband signals which, if present, can corrupt wanted information that has been mixed down around zero Hz. Causes of DC-offset are either drift in the baseband components, or DC from the mixer output caused by the LO mixing with itself or with the mixers acting as square law detectors for strong input signals
As noted above, in a zero-IF receiver, the received signal is directly down-converted from the carrier frequency to a base-band frequency, without an intermediate step where the signal is processed at an IF. This allows for a considerably lower manufacturing cost, but it is also known to suffer from problem with DC-leakage. DC-leakage occurs primarily because the carrier frequency generated at the receiver for down-mixing the signal, leaks into the receiving signal, and via the mixer results in a DC component.
In case of OFDM, the problem of DC leakage can be minimized by avoiding or nulling transmission of information on the DC sub-carrier of the OFDM signal. This means that a very small fraction of the available sub-carriers is not used, and therefore the reduction in the transmitted data rate is very small. In the event there is no frequency error, the DC component generated in the receiver will be orthogonal to the other sub-carriers. Hence, the other sub-carriers are not affected by the DC offset. However, in case of frequency offset, the sub-carriers will be frequency shifted by an amount corresponding to the frequency offset before being processed by the FFT. In this way the different sub-carriers are located properly, but the DC component generated in the receiver after the frequency correction is located at a frequency equal to the estimated frequency, but with the opposite sign.
There are currently two approaches for dealing with the low frequency part of the phase noise, sometimes referred to as the common phase error (CPE). Either, stringent design requirements are placed on the frequency synthesizer, or a number of pilot symbols on specific sub-carriers are transmitted on every OFDM symbol, so that the CPE can be estimated and subtracted from the received signal. The former approach requires that rigorous requirements be placed on the frequency generating circuitry, whereas the latter approach requires that part of the available bandwidth is not used for transmitting user data, so that the data rate possible to support is reduced.
Consequently, there is a need for a way to estimate the CPE and remove it so that the requirements on the frequency generating circuitry can be as relaxed as possible. On the other hand, there is a need for bandwidth efficient transmission, so that the CPE should be estimated without transmitting a large number of pilot symbols, that effectively only are used for estimating the CPE.
It would be advantageous to have a method and apparatus to estimate the CPE and remove it so that the requirements on the frequency generating circuitry can be as lenient as possible. On the other hand, it would be advantageous to have a method and apparatus for bandwidth efficient transmission, so that the CPE can be estimated without transmitting a large number of pilot symbols. The present invention provides such a method and apparatus.